The quantum confined Stark effect (QCSE) has given rise to several innovations in electro-optic modulators. Such modulators have many applications in communications and special purpose computer systems. The principles behind the QCSE have been more fully explained by D. A. B. Miller et al, in Physics Review, 1985, B32, p1043. Briefly though, the QCSE is a phenomenon which arises when an electric field is applied across the plane of heterostructure superlattices. In a quantum well at zero electric field, the electron and hole energy levels are defined by the well width, and the electrons and holes are strongly confined in the well layer. However, when an electric field is applied, the electrons and holes are moved apart and their energies are altered. This has the effect of shifting the absorption resonance to lower energy as well as modulating the strength of the absorption. This occurs because direct optical absorption of a photon above the band gap energy involves raising an electron from one of the valence bands and putting it in the conduction band, otherwise known as formation of an electron-hole pair. This shift in the absorption resonance, then, provides for the optical modulation of any radiation that is incident to the heterostructure.
Heretofore, several devices have been disclosed that utilize the QCSE. Examples of these devices are found in articles such as D. A. B. Miller, Quantum Wells for Optical Information Processing, Optical Engineering, Vol. 26, No.5, page 368, May 1987; Chemla et al, Electroabsorption by Stark effect on Room-temperature Excitons in GaAs/GaAlAs Mulitple Quantum Well Structures, Appl. Phys. Lett., Vol. 42, No. 10, page 864, May 15, 1983; and Chemla et al, Room Temperature Excitonic Nonlinear Absorption and Refraction in GaAs/AlGaAs Multiple Quantum Well Structures, IEEE Journal of Quantum Engineering, Vol. QE-20, No. 3, March 1984.
Most of the devices disclosed in these publications are engineered such that the heterostructures are lattice-matched to have the electron-hole transitions between the energy gaps of the valence and conduction bands of the quantum wells in direct relation. These devices are commonly called direct band gap devices and are typically fabricated by molecular beam epitaxy techniques or other similar fabrication techiques. As an example, alternate layers of Gallium Arsenide (GaAs) and Aluminum Gallium Arsenide (AlGaAs) are grown one on top of another. Because the larger band gap AlGaAs, "barriers", have both lower valence-band edges and higher conduction-band edges than the GaAs, the alternate thin layers of GaAs result in confinement of both electrons and holes within the GaAs layers, "wells". Therefore, if the AlGaAs barriers are sufficiently thick, and have a sufficiently large Al concentration so the potential barriers are high, then the penetration of the wave functions from one GaAs layer to another may be discounted for lower energy states within the GaAs layer. D. A. B. Miller et al, Electrical Field Dependence of Optical Absorption near the Band Gap of Quantum Well Structures, Physical Review B, Volume 32, No. 2, Jul. 15, 1983. In these prior art devices, an electric field moves the energy levels of the electrons and holes in the quantum well, as well as moving the electrons and holes apart, thereby altering the absorption energy (due to the former) and the strength (due to the latter) of the absorption resonance. These prior art devices had direct gap material (AlGaAs less than 44% Al) as the barrier, and did not utilize any property of the barrier other than the ability of confining the electron and holes.
The present invention, in contrast to the prior art devices, utilizes an indirect band gap heterostructure of GaAs AlAs to produce a optic modulator which operates at a greatly reduced electric field than previous devices.